Rubber tires, such as pneumatic tires, include many components, such as, for example, sidewalls. Sidewalls are continuously subjected to distortion under normal road operating conditions. The sidewalls are subjected to extensive continuous flexing and can crack under such conditions. In addition to flex cracking, sidewalls are also subjected to atmospheric chemical action such as ozone attack. The overall effect is that the sidewalls may erode and degrade. The sidewall may even separate from the tire carcass during use leading to tire failure.
To reduce the problems caused by ozone attack and flex cracking, tire manufacturers add chemical protectants to the general purpose rubbers used in tire sidewalls. The problem with these protectants is that they tend to be fugative and can cause staining when in contact with white sidewalls. In some cases, and in particular white sidewalls, polymer blends have been used to effect improvement in ozone and flex resistance.
A composition which attempts to overcome these problems is disclosed in U.S. Pat. No. 3,508,595 to Wilson and assigned to the General Tire & Rubber Company. This patent discloses a blend of chlorobutyl rubber, natural rubber, and ethylene propylene terpolymer as the protective cover for the white sidewalls used in pneumatic tire construction.
U.S. Pat. No. 3,630,974 to Ladosci, et al., assigned to Exxon Research and Engineering Company, further discloses the use of terpolymers blended with high unsaturation rubbers to enhance their ozone resistance. The patent also discloses a triblend of halobutyl rubber, ethylene propylene terpolymer and a high unsaturation rubber to improve dynamic ozone resistance and heat flex resistance. The terpolymer used in the blends comprise ethylene, propylene and a diene. The high unsaturation rubbers include natural rubber, styrene butadiene rubber and polybutadiene rubber. Among the various fillers disclosed in this patent are various "oils" along with resin, waxes, etc. The patent teaches the use of 10 parts oil per 100 parts rubber in the blends. In addition, the terpolymer used in the blends should not comprise more than 10 to 20% of diolefin.
U.S. Pat. No. 3,865,763 to Fenicek, assigned to Polysar, Limited, is primarily concerned with stabilization of halogenated butyl rubber with boron compounds. Example 5, however, appears to disclose a combination of brominated butyl rubber with both ethylene propylene rubber and styrene butadiene rubber.
U.S. Pat. No. 3,830,274 to Wesser, Jr., assigned to the Goodyear Tire and Rubber Company, discloses yet another elastomer blend for use in pneumatic tire sidewall compositions. The blend comprises an ethylene propylene nonconjugated diene terpolymer along with bromobutyl rubber and cis-1.4 polyisoprene rubber such as natural or synthetic rubber, along with a rubbery cis-1.4 polybutadiene with a specified molecular weight distribution. This blend allegedly provides substantially improved hot flex-life and carcass adhesion properties for tire sidewalls.
U.S. Pat. No. 4,224,196 discloses a sidewall composition having improved flex resistance, wherein the blend composition comprises a blend of halobutyl rubber, a highly unsaturated rubber and oil extended EPDM rubber.
Published International Application PCT/US91/05666 filed Aug. 9, 1991 discloses a tire sidewall composition comprising a single layer prepared from a blend of a halogenated copolymer of isoolefin and para-alkylstyrene with one or more general purpose rubber. While these blends exhibit superior ozone resistance and flex cracking resistance, the cost of the copolymer prohibit its broad use in tire sidewalls.
It is desirable, therefore, to reduce the amount of copolymer present in the sidewall to an economically feasible level while preserving the improved properties it offers.